Oxygen-species and nitrogen-species ions and radical gases have a very high reactivity with substances to be treated and can become means for obtaining good-quality oxide films and nitrogen films. For this reason, oxygen-species and nitrogen-species ions and radical gases are attracting attention as thin film forming means which forms insulating films in the semiconductor field and thin film forming means which improves wear resistance and corrosion resistance of working tools and the like at high temperatures. However, ions and radical gasses are very unstable and have very short life. For this reason, generated ions and radical gases often disappear before they initiate reactions with substances to be treated, thereby posing the problem that effective use of generated ions and radical gases is difficult.
The following two methods are generally known as methods of generating ions and radical gases. One is a method which involves supplying a stable molecular gas, such as oxygen gas, an oxygen compound gas, ozone gas, nitrogen gas, and a nitrogen compound gas, to a reactor, and causing the gas to discharge directly in this reactor, thereby generating ions and radical gases, what is called the discharge plasma process. The other is a method of irradiating gas with laser light and ultraviolet light or thermally decomposing a molecular gas and causing the gas to absorb light energy or heat energy, thereby generating a radical gas, what is called the light/heat absorption process.
Examples of the above-described discharge plasma process include PECVD (plasma enhanced chemical vapor deposition) process and the ion plating process. In each discharge plasma process, a high-frequency plasma and a glow discharge at low gas pressures of the order of several tens of pascals (Pa) to several thousands of pascals (Pa) are used as the discharge.
In the PECVD process and the ion plating process, a radical gas is generated by using a high-frequency plasma of the order of 2.45 GHz and a low-pressure glow discharge by the application of DC voltage. Also, a plasma is generated directly on a substrate surface, which is a substance to be treated, and an oxide thin film and a nitrogen thin film are formed by an ion gas or a radical gas in the plasma. For this reason, it is possible to bring large amounts of ion gas and radical gas species into contact with the substance to be treated, and this provides the advantage that high film forming speeds and large film thicknesses can be obtained.
On the other hand, the optical CVD process described in Patent Document 1 and the thermal CVD process described in Patent Document 2 are proposed as a generation method and a utilization method of radical gases by the light/heat absorption process.
In the optical CVD process described in Patent Document 1, a gas of a molecule having nitrogen plasma atoms having low gas pressures of the order of several hundreds of Pa to several thousands of Pa is irradiated with ultraviolet light having an optical energy of the order of several eV to ten-odd eV, whereby a nitrogen radical (N(2D), N(2P)) gas in a doublet state or a nitrogen radical (N(4S)) gas in a quartet state is generated. And the above-described nitrogen gas species is brought into contact with a substrate surface on which an oxide thin film is formed and which is heated to several hundreds of degrees, thereby the nitrogen thin film is formed.
In the thermal CVD process described in Patent Document 2, after the formation of a silicon nitride insulating film by forming a silicon nitride film on a semiconductor surface, an oxide film having a film thickness of the order of several nm is formed on this silicon nitride insulating film by dry oxidation. Incidentally, this dry oxidation is performed by installing a wafer (a substance to be treated) heated to several hundreds of degrees in a CVD chamber kept at low gas pressures of the order of several hundreds of Pa to several thousands of Pa and blowing ozone gas onto this wafer surface. That is, the ozone gas blown onto the wafer surface is thermally decomposed into an oxygen radical gas on the wafer surface. This oxygen radical gas comes into contact with silicon, which is the wafer surface, and a good-quality silicon oxide film having a film thickness of the order of several nm is formed.
Incidentally, in order to form a good-quality silicon oxide film by the above-described dry oxidation, it is necessary that the wafer surface be heated to approximately 850° C. However, if the temperature of the wafer surface becomes too high, the oxygen radical gas generated by thermal decomposition strikes against a neutral gas in the chamber before coming into contact with the wafer surface and returns to oxygen molecules. For this reason, the amount of the oxygen radical gas which comes into contact with the wafer surface decreases extremely, and it becomes impossible to obtain a prescribed oxide film thickness. Patent Document 2 describes that in order to ensure an oxygen radical gas necessary for obtaining a prescribed film thickness, an ozone gas concentration of not less than 20 vol % (430 g/m3) is necessary. Incidentally, it is generally said that oxygen radicals of a triplet oxygen radical (3P) gas and a singlet (1D) oxygen radical gas are obtained when ozone gas is decomposed.
Patent Document 1: Japanese Patent Laid-Open No. 2003-142482
Patent Document 2: Japanese Patent Laid-Open No. 2005-347679